Catalytic combustion presents a viable alternative to conventional flame combustion for addressing environmental issues for power generators such as gas turbine engines. A catalytic combustion system typically includes an active coating, such as a noble metal in a thermally stable ceramic matrix, deposited onto a metallic support. U.S. Pat. No. 4,870,824 describes a catalytic combustor wherein a first surface of a metallic conduit is coated with an active catalytic material such as palladium on a stabilized alumina (γ—Al2O3) wash-coat for causing combustion in a fuel/air plenum, while an opposed surface of the conduit has no such catalytic material and functions as a heat-removal surface to cool the conduit material.
The role of the alumina wash-coat is to provide a high surface area for exposure of the combustible mixture to the noble metal constituent. The catalyst initiates the combustion reaction at its surface at a temperature that is lower than a normal flame ignition temperature. However, once the reaction is initiated, it continues in the gas stream and persists beyond the catalyst in the form of afterburning. Simultaneously, the catalyst substrate temperature increases, resulting in an accelerated reaction that moves the reaction zone further upstream in the gas stream. The reaction must be controlled to avoid over-temperature damage to the catalyst material or to the substrate material.
The behavior of the catalyst system is primarily controlled by the properties of the active noble metal at lower temperatures. For example, the light-off temperature will be a function of the concentration of the noble metal. However, at very high temperatures, the behavior of the system will be controlled primarily by the properties of the wash-coat/support. For example, the surface area, reactivity, porosity, thermal conductivity and mechanical integrity of the wash-coat at elevated temperatures must be considered. The performance of the wash-coat may be a limiting factor in the design of a modem high temperature catalytic combustor.